JPS6349955B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a servo circuit for a rotating magnetic head drum or the like of a video signal recording/reproducing device.

When playing back VTR queues, reviews, etc. at different speeds, the playback horizontal synchronization frequency should not fall outside the locking range of the horizontal AFC circuit in the monitor receiver, causing synchronization of the playback images.
It is known that the speed servo of the head drum is performed using the reproduction horizontal synchronization signal. That is, changes in the horizontal synchronization frequency caused by changes in tape speed are compensated for by changing the scanning speed (drum rotation speed) of the rotating magnetic head. Since the vertical synchronization frequency changes by changing the drum rotation speed, the vertical synchronization signal is replaced with an external reference signal to ensure that a reproduced image is correctly formed.

Fig. 1 is a block diagram of a conventionally known drum servo system of this kind, and Figs. 2A to 2C.
The figure is a waveform diagram. In FIG. 1, during playback, the drum speed servo circuit 1 forms a speed error voltage Ev using the rotational speed detection signal FG from the frequency generator 8 provided in the drum motor 5 and the reference signal REF. Drum phase servo circuit 2
are the rotational phase detection signal PG output from the pulse generator 9 provided on the rotating drum and the reference vertical synchronization signal.
A phase error voltage Ep is also formed with VD. The phase error voltage Ep passes through the contact 3a of the changeover switch 3 and is added to the speed error voltage Ev in the servo amplifier 4, and the output of the servo amplifier 4 also controls the drum motor 5. As a result, the magnetic head 1
0 and 11 are rotated at a predetermined speed, and the tracks on the magnetic tape 12 are scanned for reproduction.

At the time of queue or review playback, the playback horizontal synchronization signal PB.H (FIG. 2A c) is supplied to the playback period detection circuit 6 (for example, mono multi circuit),
Here, a signal (FIG. 2A, b) of a predetermined pulse width synchronized with the horizontal synchronizing signal is formed. This signal b is
This is a pulse width modulation signal whose low level width changes according to the horizontal period. The signal b is supplied to the low-pass filter 7, where the DC component (FIG. 2A, c) is extracted, and then supplied to the servo amplifier 4 through the contact 3b of the changeover switch 3. Since the level of the DC signal c output from the low-pass filter 7 is obtained in proportion to the period of the reproduced horizontal synchronizing signal, if the drum motor 5 is controlled using this DC signal c, for example, when the period is shorter than a desired value, A servo loop can be configured to lower the driving voltage of the drum motor 5 and the scanning speed of the magnetic heads 10 and 11 to adjust the reproduction horizontal synchronization to the desired value. The changeover switch 3 is also switched by a control signal K from a system control circuit (not shown) of the VTR.

By the way, in high-speed playback modes such as queue and review, the heads 10 and 11 scan the tape 12 across multiple tracks, so when scanning guard bands between tracks, or when heads with different azimuths When a recorded track is scanned, the horizontal synchronization signal may be missing as shown in FIG. 2B (a). In this case, the output c of the low-pass filter 7 decreases as shown in FIG. 2B c, so that the servo function is lost.

Furthermore, when noise is superimposed on the reproduced video signal, a pseudo synchronization signal is mixed into the reproduced horizontal synchronization signal as shown in Figure 2C a, and this causes the reproduction cycle detection circuit 6 (mono multi) to be continuously triggered. As a result, a pulse signal with many high level parts as shown in FIG. 2C is formed. Therefore, the output c of the low-pass filter 7 rises significantly as shown in Fig. 2C,
As a result, the servo function is lost and the monitor TV becomes out of synchronization.

The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to prevent drum servo from being disturbed during high-speed playback due to missing synchronization signals or noise.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 3 is a partial block diagram of a portion of a drum servo system operating in a high-speed reproduction mode showing an embodiment of the present invention, and FIGS. 4A and 4B are waveform diagrams for explaining the operation.

Horizontal synchronization signal PB separated from the reproduced video signal.
H is supplied to a playback synchronization detection circuit 6, which is made up of, for example, a monomulti, through gate circuits 15 and 16, which are made up of, for example, AND gates, and is supplied here according to the playback horizontal period similar to that shown in FIG. 2A, b. A pulse width modulated signal PWM is formed. This pulse width modulation signal is supplied to a low pass filter 7 through a tri-state output circuit 17 to form a servo voltage c. This servo voltage c is applied to the drum motor 5 through the servo amplifier 4 as shown in FIG.
is supplied to

The H omission detection circuit 18 that detects omission of the horizontal synchronization signal uses, for example, a retriggerable monomulti whose time constant is slightly longer than the horizontal scanning period H. Therefore, when normal horizontal synchronizing signals are obtained continuously, the H missing detection circuit 18 continues to be triggered during the quasi-stable period at the falling edge of each horizontal synchronizing signal, so its output is always at a high level. Gate 16 is open.

When the horizontal synchronizing signal is dropped as shown by the dotted line in FIG. 4A, the output of the H dropout detection circuit 18 falls as shown in FIG. 4A and B after a short period of the horizontal period. This fall also causes the output of the gate circuit 16 to fall, so that the H missing detection circuit 18 is re-triggered and the period detection circuit 6 is also triggered as shown in FIG. 4A-C. Therefore, even if the horizontal synchronizing signal disappears, the H missing detection circuit 18 becomes a free-running multivibrator, the period detection circuit 6 is triggered, and a pseudo pulse width modulation signal is generated from its Q output.
PWM is formed.

Furthermore, when the horizontal synchronizing signal PB.H is no longer obtained, the output (rising edge) of the H missing detection circuit 18 is transmitted to the set input of the flip-flop 20 via the OR gate circuit 19, and this FF 20 is activated as shown in FIG. 4A and E. is set to That is, the fact that H omission has occurred is stored in the FF 20. The FF 20 is a set priority RS flip-flop, and the output of the period detection circuit 6 is supplied to its reset input. Therefore, the FF 20 is reset at the rising edge of the pulse width modulation signal. As mentioned above, when FF20 is set by the output of the H missing detection circuit 18, a reset input is also supplied at the same time.
0 is the set state.

The Q output of the FF 20 is supplied to the D input of the D flip-flop 21, and the FF 21 is triggered as shown in FIG. 4F at the falling edge of the pulse width modulation signal PWM. The Q output (high level) of the FF 21 is supplied to the control input of the tri-state output circuit 17.
This output circuit 17, for example, depending on the high level and low level of the input pulse width modulation signal PWM,
It is equipped with a push-pull switch circuit that flows charging current and discharging current to the capacitor in the next-stage low-pass filter 7, and the switch circuit is turned off (high impedance) by a high-level control input.
is configured to be. Therefore, even with the Q output of the FF 21, the pulse width modulation signal PWM is prohibited from being transmitted to the low-pass filter 7 through the output circuit 17 as shown in FIG. 4A, and
The output circuit 17 becomes high impedance, and the previous value of the servo voltage c output from the low-pass filter 7 is held within the low-pass filter 7.

Note that when the horizontal synchronization signal is supplied again as shown in FIG. 4A, the output of the H missing detection circuit 18 becomes high level, and the period detection circuit 6 is triggered by the horizontal synchronization signal, and the normal signal is detected from the output. A pulse width modulated signal is obtained. The FF 20 is reset by the output of the period detection circuit 6 (rising edge of PWM) as shown in FIG. 4A. Furthermore, FF21 is the fourth
It is reset at the falling edge of the pulse width modulation signal as shown in A and F.

The noise mask circuit 22 uses the horizontal synchronizing signal obtained from the gate circuit 16 or the H missing detection circuit 18.
The metastable pulse width may be about 70% of the horizontal period as shown in FIG. 4A. The Q output (D in FIG. 4) of this noise mask circuit 22 is output from the gate circuit 15.
is supplied to close this gate circuit 15. Therefore, the period detection circuit 6 and the H missing detection circuit 18 are not triggered unnecessarily due to noise in the horizontal synchronization signal section.

When noise is superimposed on the horizontal synchronizing signal as shown in FIG. 4B A, the output of the H missing detection circuit 18 becomes high level as shown in FIG. 4B B, and the period detection circuit 6 outputs a signal as shown in FIG. A short period pulse width modulation signal PWM is formed. Note that since the noise mask circuit 22 is operating, the pulse width modulation signal is output from the noise mask circuit 22 (4th B).
It is formed at approximately the same period as in Figure D). However, the servo voltage formed from this pulse width modulation signal does not function to properly operate the servo system.

The Q output of the noise mask circuit 22 is output to the counter 23.
is supplied to the enable input of this counter 23.
becomes operational. Since the input horizontal synchronizing signal is supplied to the clock input of the counter 23, the noise superimposed on the horizontal synchronizing signal in the noise mask section (the high level section in FIG. 4B and D) is counted. When the number of noises reaches a predetermined number (for example, 3), the output of the counter 23 is output to the OR gate circuit 19.
is supplied to the set input of FF 20 via.
Therefore, the FF 20 is set as shown in FIG. 4B. The FF 20 is reset at the rising edge of the pulse width modulation signal.

The Q output of FF20 is a pulse width modulation signal (fourth
Since it is read into FF21 at the falling edge of Figure B, D),
The Q output of FF21 becomes high level as shown in FIG. 4B. This turns off the output circuit 17 and interrupts the transmission of the pulse width modulation signal to the low-pass filter 7, as shown in FIG. 4B. Therefore, the output c of the low-pass filter 7 is pre-held.

In FIG. 3, the flip-flop 21 is provided to synchronize the control signal supplied to the tri-state output circuit 17 with the pulse width modulation signal, and does not need to be provided.

As described above, the present invention generates a pulse width modulated output according to the periodic variation of the horizontal synchronizing signal in the reproduced video signal, forms a servo control voltage using this modulated output, and eliminates noise or horizontal synchronization in the reproduced video signal. When a dropout of the synchronization signal is detected, the tri-state output circuit for deriving the pulse width modulation output is set to high impedance, so that the previous value of the servo control voltage is held. Therefore, the drum servo system and capstan servo system are not disturbed by synchronization signal dropout, noise, etc., and stable reproduction output can be obtained. Especially when a rotating magnetic head plays back across multiple tracks, the horizontal synchronization signal may be lost due to guard bands between tracks or tracks in different fields recorded by heads with different azimuths. , the drum servo system is not disturbed,
It is possible to stably operate the synchronization system of the monitor TV during playback at different speeds. Similarly, even if a double tone that is mistakenly recognized as a horizontal synchronization signal is mixed into the reproduced video signal, the drum servo system will not be disturbed by this.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a drum servo system of a conventional rotating magnetic head, Figs. 2A to 2C are waveform diagrams for explaining the operation of Fig. 1, and Fig. 3 shows an embodiment of the present invention. Partial block diagrams of the drum servo system, FIGS. 4A and 4B, are waveform diagrams for explaining the operation of FIG. 3. In addition, in the symbols used in the drawings, 2...
...Drum phase servo circuit, 4...Servo amplifier,
5...Drum motor, 6...Reproduction cycle detection circuit,
7...Low pass filter, 10, 11...Magnetic head, 12...Magnetic tape, 17...Output circuit,
18... H missing detection circuit, 22... Noise mask circuit, 23... Counter.

Claims (1)

[Scope of Claims] 1. A circuit that generates a pulse width modulated output in response to periodic fluctuations of a horizontal synchronizing signal in a reproduced video signal, an output circuit with a tri-state configuration to which the pulse width modulated output is supplied, and the video and a detection circuit that detects noise in the signal or omission of the horizontal synchronization signal, and forms a servo control voltage by the output of the output circuit, and when the output of the detection circuit is obtained, the output circuit A servo circuit configured to maintain a previous value of the servo control voltage by controlling the output impedance to be high.